The present invention generally relates to code generating apparatuses for generating a code which may be used as an absolute address on a recording medium or the like, and more particularly to a code generating apparatus which produces a square wave which is in phase with a sinusoidal wave having a constant frequency, and intermittently generates a sinusoidal wave as a code based on this square wave.
Presently, cassette tapes are the most popularly used recording medium. The cassette tapes are widely used in tape recorders which have a built-in radio, and cassette decks, as a recording medium on which an audio signal is recorded and from which the audio signal is reproduced. In order to enable a more convenient use of the cassette tape, some tape recorders are equipped with a music selecting function for detecting an unrecorded band between recorded music pieces so as to find the beginning of a desired recorded music piece. This music selection which is carried out by detecting the unrecorded band, is advantageous in that the existing magnetic tape can be used. However, it is impossible to detect an absolute address during such a music selection. Therefore, it was difficult to meet the demands of a random access.
Accordingly, in order to meet the demand of detecting the absolute address on the magnetic tape, various proposals have been made to record on the magnetic tape a code which indicates the absolute address. According to one proposal, a sinusoidal wave of 5 Hz to 10 Hz is intermittently recorded on the magnetic tape as a code, in a portion which is to be the unrecorded band. According to this proposed system, there will be no harmonic within the audio frequency band, because the code which is recorded has a frequency lower than the audio frequency band and is a sinusoidal wave. Thus, when the code is reproduced, the code will not be heard by the listener. This proposed system enables the detection of the absolute address in a mode in which the reproduction is carried out at a normal tape speed and in a mode in which the reproduction is carried out at a high tape speed.
However, when generating the code described above, the sinusoidal wave had to be generated so as to accurately start from a zero-crossing point and accurately terminate at a zero-crossing point. Otherwise, a harmonic was generated due to a waveform distortion in the code when the code was reproduced. In this case, the generated harmonic entered within the audio frequency band, and was audible to the listener as noise.